Remove unused letter assignment in MatchRule.

This was identified by the clang static analyser. The letter
variable is set in the various match_type switch cases, so
does not need to be initialised in the start of the while loop.

src/libespeak-ng/dictionary.c

 		unpron_ignore = word_flags & FLAG_UNPRON_TEST;
 		match_type = 0;
 		consumed = 0;
-		letter = 0;
 		letter_w = 0;
 		distance_right = -6; // used to reduce points for matches further away the current letter
 		distance_left = -2;